EUROSPEECH '97
5th European
Conference on Speech Communication and Technology

Rhodes, Greece
September 22-25, 1997

Robust GSM Speech Decoding Using the Channel Decoder's Soft Output

Tim Fingscheidt, Olaf Scheufen

Institute of Communication Systems and Data Processing, Aachen
University of Technology, Aachen, Germany

In the digital mobile radio system GSM (Global System for Mobile
Communications) there is a need for reducing the subjective effects of
residual bit errors by error concealment techniques. Due to the fact
that the standard does not specify these algorithms bit exactly, there is
room for new solutions to improve the decoding process. This
contribution presents a new approach for optimum estimation of
speech codec parameters [7] applied to the GSM system. It requires a
soft-output channel decoder (e.g. soft-output Viterbi algorithm --
SOVA [8]) providing a bit reliability information for the proposed
parameter estimation process. Additionally, a priori knowledge about
the residual redundancy in the sequence of codec parameters is
exploited. The new method includes an inherent muting mechanism
leading to a graceful degradation of speech quality in case of adverse
transmission conditions. If the channel is error free, bit exactness as
required by the GSM standard is preserved.